What is the difference between a cohort and a group?

A cohort exists for enrolment purposes. A group exists only within a course.

Cohorts are a batch(group) of users created by admin which can then be brought in "en masse" to one or more courses. So if "Class 10B" is created as a cohort by admin, then it can be selected in those courses where enrolment via cohort has been enabled. (For more information on creating and using cohorts, see: Cohorts.) Once members of a cohort have been added to a course, the individuals can be added to a group, either by using the "auto create groups" feature or by adding the members to a group.

For example, You might have students in French section 10B and German section 10A, which you want to enrol in "Principles of Language Learning" and "Principles of Linguistics " courses. You also want each section have their own teacher in each course and be unaware of the other section. You might create cohorts for the French and German sections. You would then enrol the two cohorts in the two courses. Next you would create two groups in both courses and assign individual students to the appropriate group. Note the individual students will appear in a list of students because they are members of the cohort you assigned them to.

Where can I practise with some ready made cohorts?

You can access system cohorts from Site Administration > Users > Accounts > Accounts

I want to enrol a cohort of students in all courses in a category

This is not currently possible to add a cohort to a course category and automatically enrol members in every courses at once in that course category. Please see MDL-36951 for the reasoning behind this.

You can create a special role which will allow the user to self enroll in a course. For example, all courses in the French course category are set to manual enrolment. Then assign a cohort in the context of the French course category the special role. Only special role students (all of the same cohort) can self enrol, the others will have to be manually enrolled.

You can add add a cohort without any members to a new or existing course, then add members later. This is done on a course by course basis.

What happens to course enrolments if a cohort is deleted?

If you've used cohort sync (which automatically synchronises cohort membership with course enrolments), deleting a cohort will result in all cohort members becoming unenrolled from the course(s) that the cohort was synced with. Some data associated with the members' activity in the course(s) will also be deleted.

If you've only manually enrolled a cohort in a course, then nothing happens if you then delete the cohort - users remain enrolled in the course.